200 lines
8.6 KiB
HTML
200 lines
8.6 KiB
HTML
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<title>Checkpoint a database</title>
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<!-- keywords: sqlite3_wal_checkpoint_v2 -->
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Checkpoint a database</h2>
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</div>
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<blockquote><pre>
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int sqlite3_wal_checkpoint_v2(
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sqlite3 *db, /* Database handle */
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const char *zDb, /* Name of attached database (or NULL) */
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int eMode, /* SQLITE_CHECKPOINT_* value */
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int *pnLog, /* OUT: Size of WAL log in frames */
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int *pnCkpt /* OUT: Total number of frames checkpointed */
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);
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</pre></blockquote>
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<p>
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The sqlite3_wal_checkpoint_v2(D,X,M,L,C) interface runs a checkpoint
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operation on database X of <a href="../c3ref/sqlite3.html">database connection</a> D in mode M. Status
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information is written back into integers pointed to by L and C.
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The M parameter must be a valid <a href="../c3ref/c_checkpoint_full.html">checkpoint mode</a>:</p>
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<p><dl>
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<dt>SQLITE_CHECKPOINT_PASSIVE<dd>
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Checkpoint as many frames as possible without waiting for any database
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readers or writers to finish, then sync the database file if all frames
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in the log were checkpointed. The <a href="../c3ref/busy_handler.html">busy-handler callback</a>
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is never invoked in the SQLITE_CHECKPOINT_PASSIVE mode.
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On the other hand, passive mode might leave the checkpoint unfinished
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if there are concurrent readers or writers.</p>
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<p><dt>SQLITE_CHECKPOINT_FULL<dd>
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This mode blocks (it invokes the
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<a href="../c3ref/busy_handler.html">busy-handler callback</a>) until there is no
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database writer and all readers are reading from the most recent database
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snapshot. It then checkpoints all frames in the log file and syncs the
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database file. This mode blocks new database writers while it is pending,
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but new database readers are allowed to continue unimpeded.</p>
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<p><dt>SQLITE_CHECKPOINT_RESTART<dd>
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This mode works the same way as SQLITE_CHECKPOINT_FULL with the addition
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that after checkpointing the log file it blocks (calls the
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<a href="../c3ref/busy_handler.html">busy-handler callback</a>)
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until all readers are reading from the database file only. This ensures
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that the next writer will restart the log file from the beginning.
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Like SQLITE_CHECKPOINT_FULL, this mode blocks new
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database writer attempts while it is pending, but does not impede readers.</p>
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<p><dt>SQLITE_CHECKPOINT_TRUNCATE<dd>
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This mode works the same way as SQLITE_CHECKPOINT_RESTART with the
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addition that it also truncates the log file to zero bytes just prior
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to a successful return.
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</dl></p>
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<p>If pnLog is not NULL, then *pnLog is set to the total number of frames in
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the log file or to -1 if the checkpoint could not run because
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of an error or because the database is not in <a href="../wal.html">WAL mode</a>. If pnCkpt is not
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NULL,then *pnCkpt is set to the total number of checkpointed frames in the
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log file (including any that were already checkpointed before the function
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was called) or to -1 if the checkpoint could not run due to an error or
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because the database is not in WAL mode. Note that upon successful
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completion of an SQLITE_CHECKPOINT_TRUNCATE, the log file will have been
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truncated to zero bytes and so both *pnLog and *pnCkpt will be set to zero.</p>
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<p>All calls obtain an exclusive "checkpoint" lock on the database file. If
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any other process is running a checkpoint operation at the same time, the
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lock cannot be obtained and SQLITE_BUSY is returned. Even if there is a
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busy-handler configured, it will not be invoked in this case.</p>
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<p>The SQLITE_CHECKPOINT_FULL, RESTART and TRUNCATE modes also obtain the
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exclusive "writer" lock on the database file. If the writer lock cannot be
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obtained immediately, and a busy-handler is configured, it is invoked and
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the writer lock retried until either the busy-handler returns 0 or the lock
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is successfully obtained. The busy-handler is also invoked while waiting for
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database readers as described above. If the busy-handler returns 0 before
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the writer lock is obtained or while waiting for database readers, the
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checkpoint operation proceeds from that point in the same way as
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SQLITE_CHECKPOINT_PASSIVE - checkpointing as many frames as possible
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without blocking any further. SQLITE_BUSY is returned in this case.</p>
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<p>If parameter zDb is NULL or points to a zero length string, then the
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specified operation is attempted on all WAL databases <a href="../lang_attach.html">attached</a> to
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<a href="../c3ref/sqlite3.html">database connection</a> db. In this case the
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values written to output parameters *pnLog and *pnCkpt are undefined. If
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an SQLITE_BUSY error is encountered when processing one or more of the
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attached WAL databases, the operation is still attempted on any remaining
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attached databases and SQLITE_BUSY is returned at the end. If any other
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error occurs while processing an attached database, processing is abandoned
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and the error code is returned to the caller immediately. If no error
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(SQLITE_BUSY or otherwise) is encountered while processing the attached
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databases, SQLITE_OK is returned.</p>
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<p>If database zDb is the name of an attached database that is not in WAL
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mode, SQLITE_OK is returned and both *pnLog and *pnCkpt set to -1. If
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zDb is not NULL (or a zero length string) and is not the name of any
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attached database, SQLITE_ERROR is returned to the caller.</p>
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<p>Unless it returns SQLITE_MISUSE,
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the sqlite3_wal_checkpoint_v2() interface
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sets the error information that is queried by
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<a href="../c3ref/errcode.html">sqlite3_errcode()</a> and <a href="../c3ref/errcode.html">sqlite3_errmsg()</a>.</p>
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<p>The <a href="../pragma.html#pragma_wal_checkpoint">PRAGMA wal_checkpoint</a> command can be used to invoke this interface
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from SQL.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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